The goal is to set up a mix that offers a wide stereo image, minimizes comb-filtering, and offers a quality mix regardless of listening position
June 15, 2012, by Dave Rat
As live mix engineers, the audio reinforcement systems we operate typically fall into two categories: mono or stereo.
Yes, there are the occasional opportunities to mix surround sound, and for many events, delay clusters or various fill loudspeakers are common, but for the most part it’s all about some version of mono or stereo.
While on the surface it may seem that stereo offers just a version of dual mono, there is a lot more to stereo than just two simple channels.
Stereo offers the significant advantage of allowing a perceptual horizontal source placement of the various signals. The primary challenge with mixing in stereo is avoiding situations where people on one side of the venue do not hear instruments that are panned to the other side.
To fully realize the benefits of stereo systems we need to look a bit deeper into some of the less obvious aspects of sound and human perception. Our ears not only recognize the volume and tonality of the energy that enters our ears, we also clearly perceive the direction from which the audio is radiating.
I like to think of a mix in multiple dimensions. Left to right is the X axis, near to far is the Z axis, up to down is the Y axis, plus bright to dull and loud to soft, and also, there are timing factors that can be added to further modify or perception of what is being reproduced. All of these aspects give us a diverse palette from which to add more character and clarity to the sonic experience we offer.
One of the primary challenges faced when mixing together a multitude of sources is presenting the audio such that the listener can discern each individual source, yet the sources all blend together in a cohesive and complementary manner.
Purely turning up an input signal increases audibility while simultaneously masking the audibility of other signals. It becomes quite tempting and common to constantly cycle through turning things up until the system limitations are reached. But by using a careful and a well thought-out strategy, significant improvements in clarity can be realized.
Everything Piles Up
So let’s start with what not to do. If all instruments and vocals are sent at equal volumes to both the left and right loudspeakers, everything piles up in the perceptual center.
Additionally, if both left and right loudspeakers are reproducing identical signals, there is a maximum amount of comb-filtering issues occurring everywhere except dead center. Comb-filtering is frequency dependant cancellations and summations that are most pronounced when two identical signals combine with a relative time offset between them.
To reduce audible comb-filtering, we either need to run a single mono cluster, have everyone in the room stand exactly equidistant between the left and right loudspeakers, or minimize the tonal and timing similarities between the signals reproduced by the both the left and right side of the system.
Because running a single mono PA stack leaves us with some serious coverage limitations - and only selling seats for the exact middle of the venue will put a big dent in ticket sales - focusing on sending dissimilar signals to the left and right PA sides is the way to go.
The goal is to reduce the amount of audio occurring in the perceptual center without compromising the mix on one side or the other.
Put another way, we want both the left and right sides to reproduce as different a signal as possible while still having each side well mixed and tonally balanced.
Achieving that goal can result in a minimal of buildup in the center, a wide stereo image, and a great sounding mix regardless of listener position.
The most basic way to shift the perceptual center of a source is to slightly delay one side. The advantage is that left and right tonality remains matched while shifting the center to one side or the other.
The issue is that since the actual signal being reproduced by both sides is still identical, it does not help with comb-filtering, but rather just moves the comb-filtering issues horizontally off center to one side or the other.
Utilizing two different microphone types on a single instrument, equalizing them to sound similar, and then panning them hard left and right is a very effective way to introduce differentials in the signals being reproduced. By differing mic types, I’m referring to using a dynamic mic in combination with a condenser mic or a ribbon mic.
There are several interesting things that occur when pairing mismatched mics. In addition to the inherent tonal differences, there will also be differences in volume linearity that will occur. This means that as the instrument is played, the sound will have a volume dependant “pull” to one side or the other.
Also, dynamic mics tend to have relatively heavy diaphragms made of plastic which makes them a bit slow to start and stop moving. Condenser and ribbon mics tend to have very light weight, metalized ultra-thin diaphragms that are very quick.
This “speed” factor, due to differentials in the mass of the diaphragms, introduces a different type of “pull” where the lighter weight diaphragm tends to lead and the heavier dynamic diaphragm tends to lag. Keep in mind that both mic types chosen should sound great independently to the point where either one could easily be used alone.
Mic’ing distance is also a useful tool. When mic’ing a single mono instrument with two mics, using a mic positioned very close to the instrument for the signal sent to one side of the PA and a more distant position for the mic sent to the other side is a natural way to introduce some delay to one side.
Not only is there a slight shift in the perceptual center, the added ambience of the distant mic adds a “far away” feel that tilts the perceptional angle from which the instrument radiates.
If directional mics are used, the closer mic will have more low-end than the distant mic. The phase shifts introduced by the channel EQ used to match the sound of the two mics also introduces additional differentials between the left and right signals.
Pairing a cardioid mic with a figure-8 or omnidirectional mic when dual mic’ing a sound source is yet another way to create disparities between the signals sent to the left and right PA stacks.
Room ambiance variations between the mics, as well as the non-linearities introduced by proximity effect, can nicely enhance the “stereo-ness” of the sound.
Compressors can be used to create the illusion of motion.
Busing a guitar, backing vocal, or other instrument into a stereo pair of compressors - with the thresholds and ratios at unmatched settings - will cause a volume dependant shift of the sound to one side or the other.
The compressor reaching threshold first will hold back the volume of that side, causing a perceptual swing towards the opposite side.
However, if a higher ratio is used on the compressor that hits threshold later, the sound will swing back the other way as volume is further increased.
It’s very common that both a bass mic and bass DI are utilized, yet there is no rule that both of those inputs need to be panned dead center. If you have subwoofers on an aux, send the bass DI to the subs and then try panning the DI and mic outward to free up more of the overcrowded perceptual middle ground.
The same splitting can also be applied to dual kick mics or snare top and bottom as well.
Pushing things further, several of these techniques can be combined to create multidimensional mixes. Each instrument has a panned placement in the horizontal field, a left to right relative delay distance, an ambience level, a left to right push or pull, placements that can shift with volume or tone, and numerous other possibilities.
A mix can be set up such that a guitar leans to one side or the other, and when the lead pedal is stepped on, a pair of compressors forces it to the center. Instruments can drift or shimmer between left and right while each side of the PA can sound great independently. By clearing out the typically overcrowded center sonic position, there is now a wide open space to lay in the perfectly clear lead vocal.
All of the concepts discussed here involve methods of altering the perceptual placement of instruments and vocals in the stereo field. I’ve purposely avoided discussing EQ and effects as the purpose of these techniques is neither to alter the tonal qualities of your mix nor to add effects that distract from the music.
Rather, the goal is to set up a mix that offers a wide stereo image, minimizes comb-filtering, and offers a quality mix regardless of listening position. Just simple, clean, clear natural sounds grabbed with finesse from the stage and presented to the audience in a multidimensional manner.
Dave Rat (www.daverat.com) heads up Rat Sound Systems Inc., based in Southern California, and has also been a mix engineer for more than 25 years.